quiddity
quid.di.ty \'kwid-*t-e-\ n [ML quidditas essence, lit., whatness, fr. L quid what, neut. of Xquis who - more at WHO 1a: a trifling point : QUIBBLE 1b: CROTCHET, ECCENTRICITY 2: whatever makes something to be of the type that it is : ESSENCE